It is well known that antimony-containing metal oxide catalysts containing antimony oxide and the oxide of at least one metal selected from the group consisting of iron, cobalt, nickel, manganese, cerium, uranium, tin, titanium, and copper are useful for oxidation, ammoxidation, or oxidative dehydrogenation of organic compounds. They are also useful for the production of aldehydes and acids through the oxidation of olefins and alcohols, for the production of nitriles through the ammoxidation of olefins and alcohols, and for the production of unsaturated compounds through the oxidative dehydrogenation of olefins and alcohols.
An oxide catalyst composed of antimony and at least one element selected from the group consisting of iron, cobalt and nickel is disclosed in Japanese Patent Publication No. 19111/1964. A catlyst composed of antimony oxide and tin oxide is disclosed in U.S. Pat. No. 3,152,170. A catalyst composed of antimony oxide and uranium oxide is disclosed in U.S. Pat. No. 3,308,151. Moreover, improvements in these catalysts have been proposed.
These conventional catalysts have good performance but are not necessarily satisfactory in the yield of the intended product.
It is known that the formation of an antimony-rich surface layer on the antimony-containing metal oxide catalyst is effective in improving the selectivity of the intended product. It is also known that the antimony-rich surface layer can be formed by impregnating an antimony-containing metal oxide catalyst with an antimony component. It is reported in Aso, et al., "Shokubai" (Catalyst) 21 (4) 304-306 (1979), that an antimony-rich surface layer is formed on an Fe-Sb catalyst upon calcination and that this improves the selectivity of acrolein through the oxidation of propylene. It is also reported that the selectivity of acrolein is improved by impregnating an FeSbO.sub.4 catalyst with a small amount of antimony component.
According to Y. Boudeville et al., Journal of Catalysis 58 (1) 52-60 (1979), Y. M. Cross et al., Journal of Catalysis 58 (1) 61-67 (1979), and H. J. Herniman, et al., Journal of Catalysis 58 (1) 68-73 (1979), an antimony-rich layer is formed on the surface of an Sn-Sb catalyst depending on the high temperature treatment of calcination in the course of preparation. The layer thus formed improves the selectivity of acrolein in the oxidation of propylene and the selectivity of butadiene in the oxidative dehydrogenation of butene. There is proposed in U.S. Pat. No. 4,290,920 a method for increasing the yield of the intended product by impregnating an antimony-containing oxide complex catalyst with an Sb component.
Since it is known that the formation of an antimony-rich surface layer is effective in improving the selectivity of the intended product, it has been proposed to improve the selectivity of the intended product by the impregnation of an antimony component. However, previous methods of impregnating a catalyst with an antimony component have the disadvantage that they are not easily applied to the industrial production of catalysts.
According to the above-mentioned report by Aso et al., a suspension of antimonic acid is used for the impregnation of an antimony component. It is difficult to perform uniform impregnation with a suspension except in the case where the catalyst to be impregnated has a large pore diameter. The method disclosed in U.S. Pat. No. 4,290,920 does not insure uniform impregnation.
In the production of a fluidized-bed catalyst by the impregnation of an antimony component, the desired amount of antimony is dissolved in a limited quantity of liquid corresponding to the pore volume of the catalyst, and the catalyst undergoes impregnation, drying, and calcination. In actuality, however, it is difficult to prepare a solution containing as much antimony as required, because only a small number of water-soluble antimony compounds which can be dissolved in a limited quantity of water exit. A solution of highly soluble Sb halide dissolved in hydrochloric acid causes corrosion of equipment and cannot be used industrially. On the other hand, an aqueous solution of a complex of antimony trioxide with tartaric acid or ethylene glycol is also industrially impractical, because the organic component reduces the catalyst in the subsequent calcination process and gives off an undesirable decomposition gas.
Uniform impregnation cannot be achieved with a suspension instead of a solution, because the antimony component (suspensoid) deposits on the surface of the catalyst particles and the suspending medium penetrates into the pores of the catalyst. Thus, the resulting catalyst is poor in performance and reproducibility. In addition, these impregnation methods have the common disadvantage that the production process is long and the productivity is extremely low. Such a production process typically requires the preparation of a catalyst precursor and the impregnation, drying, and calcination of the precursor.